Tigris river of blood in Isis video

Central Tikrit, Iraq. Photo / AP

New footage emerges of executions on an industrial scale after fall of Tikrit and nearby base last year.

Isis (Islamic State) has released new footage from its deadliest massacre, showing executions on an industrial scale at a military base following its attack near the Iraqi city of Tikrit last year.

The footage shows young army conscripts falling off the back of trucks at Speicher military base and later lying ready to be shot in pits or at the edge of the Tigris. At times, the water runs red with blood.

The soldiers are lined up in their hundreds and shot in shallow graves. The highest estimates put the number of dead at 1700, most of whom were Shia. At one point, the video shows an excavator being used to move piles of bodies.

Isis' capture of the Speicher base and nearby Tikrit took place during its lightning sweep across Iraq in June 2014. Government forces and allied Shia militia were only able to begin exhuming bodies in April, after a long fight to dislodge Isis militants from Tikrit.

Around 600 bodies have been located to date, with mass graves revealing corpses lain several layers deep. The massacre has been seared on the collective consciousness of Iraq's Shia population.

Many situate the atrocity within a longer history of suffering at the hands of Sunni supremacists and the Baathist regime of former President Saddam Hussein.

Combined with a call by the country's top Shia cleric, Grand Ayatollah Ali al-Sistani, for Iraqis to take up arms, the Speicher massacre played a key role in the mass recruitment of Shia volunteers to fight the jihadists.

A coalition of mostly Shia militias known as the Hashed al-Sha'abi is leading the counter-attack against Isis in Sunni-majority Anbar province. Isis seized Anbar's capital, Ramadi, two months ago, extending its control over the Euphrates River valley west of Baghdad and dealing a major blow to the Government.

The Hashed launched an offensive to retake the Anbar city of Fallujah last week, but faced stiff resistance from Isis fighters. Fallujah saw the fiercest fighting of the United States occupation that followed Washington's 2003 invasion to topple Hussein, and has also been a centre of Sunni hostility to the Shia-led Government in Baghdad.

The empowerment of the Hashed as a fighting force has left Sunni communities on edge, with many fearful that they will now face revenge attacks.